An associative polymer contains pendant groups capable of forming non-specific “associations” with other groups in the polymer or other materials in the medium in which the polymer is present. Generally the pendant group has both hydrophobic and hydrophilic regions and the associations are generally based on hydrophobic interactions. Hydrogen bonding associations between hydrophilic groups have also been seen under some pH conditions. According to theory, such associations result in thickening by the formation of interpolymer networks above a critical polymer overlap concentration.
Hydrophobically modified alkali-swellable or alkali soluble emulsion polymers, conventionally referred to as HASE polymers, are associative polymers that are typically polymerized as stable emulsions at low pH (pH<4.5) but become water swellable or soluble at near neutral to neutral pH (pH>5.5-7). Typical HASE polymers are vinyl addition copolymers of pH sensitive or hydrophilic monomers, hydrophobic monomers and an “associative monomer”. The associative monomer has a polymerizable end group, a hydrophilic midsection and a hydrophobic end group. An extensive review of HASE polymers is found in Gregory D. Shay, Chapter 25, “Alkali-Swellable and Alkali-Soluble Thickener Technology A Review”, Polymers in Aqueous Media—Performance Through Association, Advances in Chemistry Series 223, J. Edward Glass (ed.), ACS, pp. 457-494, Division Polymeric Materials, Washington, D.C. (1989), the relevant disclosures of which are incorporated herein by reference.
Conventional HASE polymers generally contain a single associative monomer. Conventional HASE polymers may be derived from associative monomers having a hydrophobic end group which is substantially a single hydrocarbon moiety or from associative monomers having hydrophobic end groups which are predominantly mixtures of alkyl groups having molecular formulas differing by about 2 carbon atoms with minor amounts of alkyl groups differing by up to about 6 carbon atoms, for example, alkyl groups derived from some natural fatty materials.
Conventional HASE polymers have been used as rheology modifiers, emulsifiers, stabilizers, solubilizers and pigment grinding additives in industrial applications. However, HASE polymers have found limited utility as rheology modifiers in aqueous formulations, because the thickening ability of HASE polymers tends to be relatively low at practical use levels of about 1% or less. Increasing the amount of HASE polymer not only is economically undesirable, but highly viscous HASE polymer solutions can be difficult to handle during manufacturing processes on a commercial scale. In addition, increased thickening often occurs at the expense of the optical clarity of the final product, which is undesirable in certain personal care applications especially for hair care. Consequently, the HASE polymers are conventionally combined with additional rheology modifying polymers.
Some prior attempts have been made to enhance the thickening ability of associative polymers and improve their aqueous thickener performance. For example, U.S. Pat. No. 5,916,967 describes enhancing the thickening ability of associative polymers by mixing the polymer with two or more surfactants. Similarly, surfactant-thickener interactions are disclosed by C. E. Jones in “A Study of the Interaction of Hydrophobically-Modified Polyols with Surfactants”, Proceedings of the 4th World Surfactants Congress, CESIO, Barcelona, 2, 439-450 (1996) and by P. Reeve in “Tailoring the Properties of Polymeric Rheology Modifiers to the Characteristics and Requirements of Personal Care Formulations”, Proceedings of International Federation of Society of Cosmetic Chemists, IFSCC, Budapest, 337-346 (April 1997).
An approach for improving the thickening properties of aqueous solutions using macromonomer-derived associative polymers employing surfactants as co-thickeners is disclosed in U.S. Pat. No. 5,292,843. European Patent Application No. 1,038,892A2 describes adding a mixture of at least one multiphobe and at least one monophobe compound (as an additive) particularly to improve the viscosity stability of an aqueous system containing at least one associative thickener. A method of suppressing the viscosity of HASE polymers in aqueous compositions by complexation of the hydrophobic moieties of the polymer with cyclodextrin compounds (capping agent additive) is disclosed in U.S. Pat. No. 5,137,571 and U.S. Pat. No. 6,063,857.
There is an ongoing, unresolved need and desire for an associative polymer having improved rheological and aesthetic properties in an aqueous media. The multi-purpose alkali-swellable associative polymers (ASAP) of the present invention surprisingly provide such desirable rheological and aesthetic properties in aqueous media.